The invention relates to secondary cells, in particular rechargeable lithium batteries, and it relates more particularly to making carbon fiber electrodes for such cells.
In lithium secondary cells, it is known that a carbon-based material can be used as the anode into which lithium ions can become inserted during charging and from which they are de-inserted while discharging.
Usually, the carbon-containing material is presented in the form of a carbon or graphite powder bonded with a binder and deposited on an electrically conductive substrate such as a sheet of copper or other conductive metal. Document EP 0 765 505-A1 illustrates the state of the art, for example.
A drawback of that known technique lies in the use of solvents for mixing the carbon-containing powder with a binder, which solvents need to be recovered and regenerated. In addition, the binders in common use deteriorate at temperatures that are relatively low, typically above 50xc2x0 C., running the risk of spoiling the electrode. Furthermore, options for shaping the complex constituted by the metal substrate covered in one or two layers of bonded-together carbon-containing particles in a binder without damage are restricted, thereby limiting the electrode shapes that can be made.
Proposals have also been made to use a carbon-containing material in the form of carbon fibers, in particular in the form of a woven or non-woven fabric of carbon fibers. Reference can be made to document EP-A-0 629 011 which describes an electrode made up of a sheet of uni-direction fibers secured by adhesive on a foil of electrically conducive material. Constraints on shaping exists as in the preceding case.
There is also a problem that exists with using a carbon-containing material in the form of graphite: this is the risk of exfoliation, i.e. the carbon-structure becoming partially destroyed after successive charge and discharge cycles.
An object of the invention is to provide a carbon fiber electrode making it possible to eliminate or at least greatly attenuate that risk. Another object of the invention is to provide a carbon fiber electrode is easy to shape. The invention also has the object of providing a carbon fiber electrode having improved insertion and de-insertion capacity for lithium ions when used in lithium secondary cells. In this particular application, another object of the invention is to provide a carbon fiber electrode in a form that makes it possible to introduce agents for doping lithium-affinity in electron and/or steric hindrance terms.
In one aspect, the invention provides a secondary cell electrode comprising a carbon fiber substrate in which the carbon of the fibers has a mean lattice surface spacing (d002) of not less than 0.36 nanometers (nm), and a crystallite size such that the mean height (Lc) lies in the range 0.7 nm to 3 nm and the mean width (La) lies in the range 2 nm to 6 nm, and presents total porosity lying in the range 10% to 30%.
The lattice surface spacing (d002) is a measure of the distance between two consecutive graphene sheets within the structure of the carbon, as obtained by scattering X-rays on the substrate. The dimensions Lc and La are measured in the same manner, as described in detail below.
Preferably, the mean lattice surface spacing (d002) is not less than 0.38 nm, while the size of the crystallites is such that the mean height (Lc) is equal to about 1 nm and the mean width (La) is equal to about 3 nm, and the total porosity of the carbon fibers is equal to about 20%.
Thus, a characteristic of the invention lies in the use of carbon fibers in which the lattice surface spacing (d002) is greater than that of graphite (typically 0.3354 nm). The risk of exfoliation by repeated insertions and de-insertions of lithium ions between the sheets of the carbon structure is thus greatly reduced, or even eliminated. In addition, the small dimensions of the crystallites and the porosity thereof provide spaces for receiving lithium ions.
The carbon fibers come from a precursor that gives rise to carbon that is xe2x80x9chardxe2x80x9d, i.e. non-graphitizable, such as a precursor selected from cellulose precursors, phenolic precursors, and isotropic pitch precursors. Such a xe2x80x9chardxe2x80x9d carbon presents a structure and a microtexture that are much less organized than that of graphite, which also favors lithium ion insertion and de-insertion, and which makes it possible to obtain lithium batteries of high capacity.
The carbon fibers are preferably derived from a cellulose precursor, such as a rayon or a rayon staple in which carbon black may have been incorporated.
Also preferably, the carbon fiber substrate is a two-dimensional fabric, in particular a woven cloth, having a cellulose precursor and in particular a rayon precursor, and which is not bonded to a conductive metal support. This avoids the problems that result when using binders that can be damaged by being heated, and possible shaping of the electrode, e.g. by being rolled up, is facilitated.
In another aspect, the invention provides a method of manufacturing an electrode for a secondary cell, the method comprising the steps which consist in:
preparing a two-dimensional fabric of carbon-precursor fibers selected from cellulose precursors, phenolic precursors, and isotropic pitch precursors; and
performing heat treatment to transform the precursor into carbon, in which the lattice surface spacing (d002) is not less than 0.36 nm, preferably not less than 0.38 nm, the crystallite size is such that the mean height (Lc) lies in the range 0.7 nm to 3 nm, preferably equal to about 1 nm, and the mean width (La) lies in the range 2 nm to 6 nm, preferably equal to about 2 nm, and the total porosity of the carbon fibers lies in the range 10% to 30%, preferably equal to about 20%.
The heat treatment for transforming the precursor into carbon is performed at a temperature that preferably lies in the range 700xc2x0 C. to 1300xc2x0 C., i.e. well below that required for graphitization, which is typically 2800xc2x0 C. When using a rayon precursor, the heat treatment temperature advantageously lies in the range 900xc2x0 C. to 1000xc2x0 C., and the time spent at that temperature preferably lies in the range 0.1 minutes (min) to 1.5 min.
By a suitable choice of carbon precursor and of heat treatment, it is possible to provide a carbon that has sheets containing a large number of defects of the sp3 hybridization type and having crystallites of very small size. This gives rise firstly to a smaller risk of exfoliation because the carbon structure has a certain amount of xe2x80x9celasticityxe2x80x9d, and secondly to great capacity for insertion and de-insertion of lithium ions in particular because of the larger number of pores contained within the texture of the fiber.
The carbon fiber fabric can be obtained using a two-dimensional fabric such as a woven cloth, a felt, a knit, a uni-directional sheet, or a plurality of uni-directional sheets that are superposed.
According to another aspect of the invention, in a secondary cell having an anode and a cathode enabling lithium ions to be inserted and de-inserted, and also having an electrolyte, the anode is constituted by a carbon fiber substrate made as described above.